Our objective is to synthesize a series of novel enkephalins having alpha, beta-unsaturated (dehydro)phenylalanine, tyrosine and/or alanine residues replacing the corresponding L-residues. New methods, designed by ourselves and others, will be used to introduce the unsaturated sites. An attempt will be made to correlate the bioactivity of these opiate peptides with double bond configurations and molecular conformations. The unsaturation is expected to increase both the binding to the bioreceptor and the stability toward enzymolysis of the enkephalins.